Pressed paper products have hitherto been universally used as various forms of containers in the field of packaging and containers. There are one-piece pressed paper items such as paper trays or paper dishes, and two-piece containers such as paper cups formed of a body member and a bottom member.
Pressed paper pieces formed by deep drawing a blank that is primarily made of paper lack malleability and ductility unlike metals and the blank tends to wrinkle in upright portions along the peripheral edge, as the paper material is gathered together with the reduction in diameter during the deep drawing process. FIG. 6 shows a common structure of a two-piece paper cup consisting of a body member X and a bottom member Y. The bottom member, which is the pressed paper piece, is formed by deep drawing. As shown in the drawing, the lower end of the body member is folded back, and the bottom member is jointed thereto by bonding, with the side wall of the bottom member being sandwiched between the folded back portions. Any wrinkles that are so large as to be folded over in the side wall will cause an insufficient bond because of gaps in the bonded surfaces. Such an insufficient bond would cause no problem if the paper cup is disposed after holding a content liquid that is immediately consumed. The paper cup, though, could not withstand a long-term use as a container. The following techniques are conventional methods of reducing wrinkles in pressed paper pieces for ensuring that a good bond is formed.
Patent Document 1 discloses a method of producing a bottom piece for paper cups, with an aim to produce a cup bottom such that no wrinkles that are so large as to be folded over are formed when compressing the rim of the bottom piece. The method includes: applying a clamping force to an outer peripheral portion of a thick circular paper stock; pressing a central portion of the paper stock into a circular opening with a smaller diameter than that of the paper stock while maintaining the clamping force to prevent displacement of the outer peripheral portion, thereby to stretch the paper stock; forming the rim, after the previous step of stretching the paper stock, by reducing the clamping force gradually and letting the outer peripheral portion enter the opening and bend so that it extends substantially orthogonal to the central portion.
Patent Document 2 aims at providing a leakage-proof bottom paper piece fitted and bonded to a body section of a paper cup, and shows a configuration wherein a through hole (female mold) is formed in a mold section, and an annular pressing member driven by a coil spring is arranged to press a bent portion of the bottom paper piece against a stepped surface of the mold section. With the bent portion being pressed by the pressing member, a punch (male mold) is activated to deep draw the bottom paper piece. With this configuration, as the bottom piece is gradually deep drawn while an outer side of its bent portion is held down, narrow pleats-like, easily-crushable folds are formed in the bent portion. As a result, when joined by applying pressure, the bottom piece forms a favorable tight bond with the body section, and thus highly water-proof paper cup can be obtained.
Patent Document 1: Japanese Patent Application Laid-open No. 2000-238150 “Paper cup bottom and method and apparatus for forming same”, published on Sep. 5, 2000
Patent Document 2: Japanese Patent Application Laid-open No. 2001-270013 “Apparatus and method for forming bottom paper sheet of paper cup” published on Oct. 2, 2001